Environments with high levels of ionizing radiation create special design challenges. A single charged particle may knock thousands of electrons loose, causing electronic noise and signal spikes. In the case of digital circuits, this may cause results which are inaccurate or unintelligible. This is a particularly serious problem in the design of artificial satellites, spacecraft, military aircraft, nuclear power stations, and nuclear weapons. In order to ensure the proper operation of such systems, manufacturers of integrated circuits and sensors intended for the aerospace markets employ various methods of radiation hardening. The resulting systems are said to be radiation-hardened.
Typical sources of exposure of electronics to ionizing radiation are solar wind and the Van Allen radiation belts for satellites, nuclear reactors in power plants for sensors and control circuits, residual radiation from isotopes in chip packaging materials, cosmic radiation for both high-altitude airplanes and satellites, and nuclear explosions for potentially all military and civilian electronics.
Two types of space radiation are of particular concern for spacecraft electronics designers. The first, known as the total ionizing dose, represents the cumulative effect of many particles hitting a device throughout the course of its mission life, slowly degrading the device until it ultimately fails. The second involves high-energy particles that penetrate deep into materials and components, leaving a temporary trail of free charge carriers in their wake. If these particles hit vulnerable spots in the circuit, they may produce adverse effects, described generally as single-event effects.
One type of electronic component often found aboard a satellite is the complementary metal-oxide semiconductor (CMOS) integrated circuit. As commercial CMOS processes have advanced, the inherent radiation resistance of these devices has improved. For example, the current that flows through CMOS transistors is governed by a low-voltage gate over each device, isolated by a layer of oxide. These insulating layers may develop a charge after long exposure to ionizing radiation, and this charge may affect the flow of current through the device. As circuits have shrunk, however, the thicknesses of these insulating layers have decreased, presenting less opportunity for charge buildup.
More problematic are the radiation-induced increases in leakage current. Leakage also increases the amount of current flowing in the circuit, when the device is in a quiescent state. Such an increase, multiplied by the tens of millions of switches in each circuit, may drive up power consumption. In an extreme case, the isolation between discrete components may also be lost, rendering the circuit useless.